Added regression test in tests/advection_2d_square that checks sum of t and q at two gauges
Added tests/advection_2d_annulus
Added tests/acoustics_2d_radial
Added regression_tests.py to examples/advection_2d_square:  compares to gallery version...
See comments in file.
Also fixed bugs in clawpack/clawutil/src/python/clawutil/imagediff.py
on branch regression_imagediff
Added tests/run_tests.py and improved advection_2d_square/regression_tests.py
modified tests/advection_2d_square to save and read regression results rather than hardwiring into script
Changed other tests to read in archived data
added tests/Makefile and more to README
improved comments
Merge branch 'master' into regression

PR #69 is out of date relative to more recent changes -- merging those in to
check that regression tests still work.

* master:
  changed to use dt_max in setrun.py to avoid huge cfl numbers when vt=0
  changed name of unused variable dtv2 -> dt_max. Add line to use it in tick - default in setrunn is 1e98 so shouldn't change anything unless actually set it in setrun.py:
  added dimension statement for aux - safer this way
  tstart var in conck should have been changed when variable replaced by either tstart_thisrun or t0
  missing maux and dimension statement
  fixed README file again
  fixed README file
  fixed examples/acoustics_2d_radial to add .m files from @donnaaboise PR #65 and fix 1drad subdirectory to properly use new classic code
  missing aux argument in qinit
  change README.txt to .rst. Most files only have titles, little description yet.
Improved regression tests to make it easier to adapt to new problems
created files in examples/ top level directory to do regression tests in all subdirectories, comparing against gallery figures.
reverted examples/advection_2d_swirl/setrun.py to proper version
added .travis.yml to see if this works with regression tests in tests/...
May have to update clawpack/clawpack first.
Added examples/Makefile for regression tests
Merge pull request #69 from rjleveque/regression

New attempt at regression tests in tests/ and examples/